A great deal of interest has been focused on solid state light sources (SSLSs), such as light emitting diodes (LEDs) and lasers, in particular those that emit light in the blue and deep ultraviolet (UV) wavelengths. These devices may be capable of being incorporated into various applications, including solid-state lighting, biochemical detection, high-density data storage, and the like. UV LEDs are mainly fabricated using group III nitride heterostructures. In these materials, achieving a high level of p-type doping required for efficient LED operation presents a significant problem due to high ionization energy of acceptor impurities.
Prior approaches have proposed various designs for LEDs. In one approach, the proposed structure includes an additional SiO2/semiconductor interface at which a hole accumulation layer is formed. The holes need to tunnel through the dielectric to reach the LED active region. Although the hole accumulation layer in this design may help increase the hole concentration significantly, the concentration of holes tunneling through the dielectric is significantly lower than that in the accumulation layer. Secondly, adding the dielectric layer substantially increases the turn-on voltage of the device.
In another approach, an LED design includes a tunnel junction formed between n- and p-AlGaN layers. In this design, a high electron concentration in the top AlGaN layer causes carrier tunneling through the junction and reduces the lateral spreading resistance. However, this design does not allow for significant improvement in the hole injection into the LED active region. In still another approach, an LED design includes a hole acceleration layer. In particular, the LED contains an additional p-GaN layer and p-AlGaN barrier forming a region with a strong electric field enhancing the hole emission over the barrier. This approach is limited by an excessive voltage drop across the additional barrier and an insignificant increase in the hole injection into the LED active region.